The light olefins are important raw materials for petrochemicals. Their general production methods include natural gas steam cracking, naphtha or light cycle oil thermal cracking in a tubular furnace, heavier hydrocarbon thermal cracking on solid heat carrier, and catalytic conversion of light alcohol's. The conventional catalytic crackers also produce light olefins as by-products with a yield lower than 15 wt % of the charge during the production of gasoline and light cycle oil.
A survey of the literature shows that there are approximately three types of catalysts used for producing light olefins by cracking petroleum hydrocarbons. The first type catalyst is oxides supported metal catalysts, wherein the support can be SiO.sub.2, Al.sub.2 O.sub.3, or other oxides and the metal components are mainly selected from elements of IIB, VB, VIIB, and VIII groups, as disclosed in U.S. Pat. No. 3,541,179; 3,647,682; DD 225,135; and SU 1,214,726. As the supported metal possesses dehydrogenation ability, it also acceralates the condensing and coking reaction during the reaction process of cracking. Hence, this type catalysts can only be used for processing light feed stocks (&lt;220.degree. C.)
The second type catalyst is oxide composites, such as a composite comprising mainly ZrO.sub.2 and/or HfO.sub.2 and also Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, MnO and/or Fe.sub.2 O.sub.3 and alkali or alkaline metal oxides, as disclosed in U.S. Pat. No. 3,725,495 and 3,839,485. In another example, the potassium vanadate, potassium stannate, or potassium niobate was used as catalyst for cracking gasoline to obtain approximately 56 wt % of light olefins including 36.5 wt % of ethylene and 12.5 wt % of propylene, as disclosed in SU 523,133; 487,927 and 410,073. In further examples, the SiO.sub.2.Al.sub.2 O.sub.3 catalysts containing minor content of Fe.sub.2 O.sub.3, TiO.sub.2, CaO, MgO, Na.sub.2 O, or K.sub.2 O were also used in cracking various hydrocarbon fractions, as disclosed in SU 550,173; 559,946. The rather commonly used oxide composite was amorphous SiO.sub.2.Al.sub.2 O.sub.3, as used in DD 152,356 as a catalyst for cracking various liquid hydrocarbon or hydrocarbon fractions (including gasoline, kerosene, gas oil, or vacuum distillates) at 600.degree.-800.degree. C. to produce light olefin with a C.sub.2 -C.sub.4 olefin yield of 40-55 wt %.
Along with the extensive application of zeolites in petrochemical and petroleum refining industry, the third type catalyst, i.e., zeolite-containing catalysts especially ZSM-5 zeolite-containing catalysts, were reported in a large number of literature. This type catalyst can be solely used or used as an additive in catalytic crackers or incorporated in cracking catalysts. JP 60-224,428 disclosed a cracking catalyst using ZSM-5 zeolite as active component and alumina as matrix, which was used at 600.degree.-750.degree. C. to crack a feed stock of C.sub.5 -C.sub.25 paraffinic hydrocarbons with a C.sub.2 -C.sub.4 olefin yield of roughly 30%. In U.S. Pat. No. 4,309,280, HZSM-5 zeolite was added directly into the catalytic cracker by a weight percent of 0.01-1 based on the weight of the catalyst. In U.S. Pat. No. 3,758,403, a catalyst comprising both ZSM-5 zeolite and a large pore zeolite (e.g. X type or Y type) as active components with a ratio of 1:10-3:1 displayed a performance in simultaneously enhancing gasoline octane number and increasing C.sub.3 -C.sub.4 olefin yield to about 10 wt %. The present applicants also disclosed in CN 1004878B, U.S. Pat. No. 4,908,053, and CN 1043520A, catalysts using mixed ZSM- 5 zeolite and Y zeolite as active components, which exhibited performance in simultaneously enhancing gasoline octane and C.sub.2 -C.sub.4 olefin especially C.sub.3 -C.sub.4 olefin yield at 500.degree.-650.degree. C.
The object of the present invention is to provide a novel zeolite-containing catalyst possessing higher hydrothermal stability, conversion ability and C.sub.2 -C.sub.4 olefin yield during cracking reaction.
The other objects of the present invention can be learned from the content of the specification of the present invention including the claims.